The manufacture of electronic components, such as semiconductor wafers, liquid crystal displays, light emitting diodes and solar cells typically requires nitrogen containing ten parts per billion (ppb) or less of several contaminants, including carbon monoxide, hydrogen, and oxygen. Nitrogen containing contaminants at these levels is referred to as ultra-high purity nitrogen. Ultra-high purity nitrogen is used, for example, to generate a contaminant-free atmosphere during various electronic component processing steps, thereby minimizing the number of defects in the product manufactured.
The base material utilized in the production of ultra-high purity nitrogen is air. With reference to FIG. 1, a conventional system 100 is depicted. Air is introduced into compressor 110 where it is compressed to a pressure ranging from 35 psig to 200 psig. The resulting high pressure air stream is fed to an adsorption system 120, which contains two or more beds arranged in parallel. Adsorption system 120 typically operates at or near ambient temperature and removes high boiling point contaminants such as water and carbon dioxide. The resulting purified air is routed to a cryogenic air separation unit 130 that contains, for example, at least one distillation column and removes the preponderance of moderate boiling point contaminants such as oxygen. The nitrogen stream which exits the air separation unit is a conventional purity nitrogen stream and typically contains 1-10 parts per million (ppm) oxygen, 1-10 ppm carbon monoxide and 1-10 ppm hydrogen. The air separation unit also produces an oxygen-containing stream that may be utilized in part to remove contaminants from adsorption system 120.
The conventional purity nitrogen stream is further purified in a chemical adsorption based gas purifier 140. This gas purifier typically contains a chemical adsorbent that is based on a metal, such as nickel, and reacts with and/or adsorbs any residual oxygen, hydrogen and carbon monoxide. Contaminants that have reacted with or adsorbed on the metal based catalyst are removed in a regeneration step by reaction and thermal desorption using a heated hydrogen/ultra-high purity nitrogen mixture. Typically, 1-10% of the ultra-high purity nitrogen stream is employed for this purpose. The nitrogen/hydrogen/contaminant mixture exiting the chemical adsorption based purification system 140 is discarded.
The ultra-high purity nitrogen stream generated in the purifier is then routed to filter system 150 to remove any particulates, and thereafter the ultra-high purity nitrogen stream is routed to the point of use.
The contaminant level in the conventional purity nitrogen stream exiting the air separation unit 130 can be compromised, for example, by air entering the system before the stream reaches the gas purifier 140. A high concentration of some contaminants/impurities, such as oxygen, can create an exothermic reaction. As a result, the chemical adsorbent in gas purifier 140 reaches temperatures exceeding a predetermined value, typically ranging between 120° F. and 400° F. This exothermic reaction can result in the destruction of the chemical adsorbent, as well as the release of aqueous corrosives which can destroy the gas purifier and contaminate downstream piping.
To prevent such a potentially catastrophic event from occurring, the gas purifier 140 is taken off-line and ultra-high purity nitrogen flow to the end user is discontinued if excessive contaminant levels are detected in the conventional purity nitrogen stream. The gas purifier 140 is isolated from the conventional purity nitrogen stream and the end user does not receive ultra-high purity nitrogen. Thus, a substantial economic loss is incurred.
Various attempts have been made to monitor the contaminant level in the incoming gaseous stream from the air separation unit, so as not to allow the purifier to exceed a specified temperature point.
Specifically, the related art systems, are designed to include an oxygen sensor or otherwise a temperature detection mechanism to determine the level of contaminant in the nitrogen stream. Billingham et al in U.S. Pat. No. 7,097,689 B2 discloses gas sampling both upstream and downstream of the purifier. The gas streams are combined and sent to a single oxygen analyzer. If the oxygen levels are above a selected level, the gas analyzer will alert that the upstream or downstream gases contain too much oxygen.
U.S. Pat. No. 6,824,752 B1 to Terbot et al is directed to a system for protecting a purifier from damage that includes passing a stream of impure gas through a catalyst bed and measuring the temperature difference before and after the catalyzed bed reaction through a data analyzer to determine the impurity level in the gas.
U.S. Pat. No. 6,168,645 B1 to Succi et al relates to a safety device coupled either to the unpurified gas inlet line or the purified output line or both. This safety device develops an alarm signal when gas contaminants exceed a given concentration level for a period of time.
U.S. Pat. No. 6,068,685 discloses a gas purifier including a getter column having a metallic vessel and a containment wall extending between the inlet and the outlet. The getter material purifies gas flowing therethrough by adsorbing impurities therefrom. A first temperature sensor is located in the top portion of the getter material and a second temperature sensor is located in the bottom portion of the getter material to rapidly detect the onset of an exothermic reaction which indicates the presence of excess impurities in the gas which is to be purified.
The conventional systems described above, which are designed to prevent excessive contaminants such as oxygen from reaching the gas purifier, lack in certain respects. For example, oxygen analyzers/sensors typically requires 5-10 seconds to respond to an oxygen reading. Therefore, the conventional purity nitrogen stream could reach the gas purifier before an excessive oxygen level can be detected. Such an occurrence would necessitate the isolation of the gas purifier (i.e., take it off-line), when elevated oxygen levels are detected, causing discontinuation in the flow of ultra-high purity nitrogen to the end user. The loss of nitrogen flow can result in electronic component damage to the end user costing large amounts of monies.
In ultra-high purity gas production plants, it is increasingly desirable to design a system which prevents a gas stream containing excessive contaminant levels from reaching a chemical adsorption-based gas purifier, while maintaining continuous gas supply to the end user. Specifically, it is an object of the invention to prevent nitrogen containing excessive oxygen levels from reaching a chemisorbent-based gas purifier while maintaining nitrogen supply to the end user.